For field devices of process automation for use in explosion endangered areas, an Ex-secure embodiment of power- and data transmission is of great importance. For this, the power transmission between the electronics unit and the sensor of a measuring device occurs frequently via a capacitive or inductive interface. For communication between the primary side and the secondary side, often Ex-secure optocouplers are used. These are, however, costly and require additional space in the electronics housing.
An alternative to optocouplers is to modulate the transmitted signals. For transmission of data from a primary side to a secondary side via a conductive or inductive interface, most often, the frequency of the signal is modulated. For this, a primary side electronics unit produces a signal with a first frequency or with a second frequency as a function of whether a logical zero or a logical one is to be transmitted. The signal serves simultaneously for energy transmission. For communication from the secondary side to the primary side, for example, the electrical current requirement of the secondary side is modulated. This form of energy- and data transmission is disclosed, for example, in Offenlegungsschrift DE 10 2006 051 900, A1.
A further method for communication from the secondary side to the primary side by means of amplitude modulation is described, for example, in Offenlegungsschrift WO 2008/135402 A1. The amplitude modulated signal is produced by a class-E amplifier. Amplitude shift keying in the output signal is achieved, in such case, either by varying the operating frequency of the class-E amplifier, or, in the case of constant operating frequency, by shifting the resonant frequency of the amplifier by means of an addable load.
Disadvantageous in the case of amplitude modulation is the relatively high disturbance susceptibility relative to in-coupled signals.